A. Technical Field
The present invention relates to a water-absorbent resin which is, for example, used favorably for sanitary materials (e.g. disposable diapers, sanitary napkins, so-called incontinent pads). More specifically, the present invention relates to a process in which a fine powdery water-absorbent resin, which is difficult to use if it is left as it is, is utilized to efficiently produce a particulate agglomerate or particulate water-absorbing material that is appropriate for the use.
B. Background Art
The fine powdery water-absorbent resin is generated in a process for production of a water-absorbent resin. For example, when a blocky or sheet, water-absorbent resin is pulverized to produce a water-absorbent resin product, particles having become finer than the desired particle diameter range are generated. Besides, in a process for production of a water-absorbent resin product, there is a case where a fine powder, which is inappropriate as a product, is generated due to such as collision of particles with each other or with apparatus. In addition, in the case of the reversed-phase suspension polymerization, a fine powder not larger than a definite particle diameter is formed as a by-product during the polymerization.
Arts by agglomeration are proposed as arts for recovering such a fine powder of a water-absorbent resin to recycle it. As to a binder for the water-absorbent resin during the agglomeration, generally, water or an aqueous liquid is favorable from the viewpoint of such as efficiency, safety, and production costs. Thus, also in nearly all the above methods, the aqueous liquid which plays a role like the binder is added to the fine powder.
However, a water-absorbent resin, particularly a fine powdery one, has a large surface area and is therefore so fast in absorption rate that it is difficult to uniformly add the aqueous liquid thereto. In addition, the use of such as insoluble inorganic powder as a mixing assistant for the aqueous liquid, generally, has not only problems of costs, but also problems of dust generation and deterioration of agglomeration strength and other physical properties.
Patent document 1 below discloses a method including the steps of: spraying a fine powder of a water-absorbent resin with water or an aqueous solution of a water-soluble polymer to thereby wet the surfaces of the fine powder of the water-absorbent resin; and then compression-molding it into a sheet shape; and then pulverizing the resultant molding to thereby obtain an agglomerate material.
Patent document 2 below discloses a method including the steps of: mixing a fine powder of a water-absorbent resin with water (deionized water) to thereby wet the fine powder of the water-absorbent resin to thus obtain an amorphous or homogeneous gel having a water content (ratio of water to the total weight of the water-absorbent resin and water) of 62 to 80%; and then cutting and drying the gel to thereby obtain an agglomerate material. Furthermore, patent documents 3 and 4 below discloses a method including the steps of: stir-mixing a fine powder of a water-absorbent resin with water or a physiological saline solution to thereby form viscous agglomerates having a water content of 67%; and then forming them into a sheet by low-temperature extrusion coating or spreading. In addition, patent document 5 below discloses a method including the steps of: using a small-sized turbilizer to add inorganic fine particles to a fine powder of a water-absorbent resin and further an aqueous solution of a crosslinking agent to this fine powder, thereby obtaining a hydrous material of a water-absorbent resin having a water content of 17%; and then further applying mechanical compression to this hydrous material to process it into a sheet shape; and then drying and pulverizing the resultant sheet to thereby obtain an agglomerate material.                [Patent Document 1]                    JP-A-113557/1998 (Kokai)                        [Patent Document 2]                    U.S. Pat. No. 4,950,692                        [Patent Document 3]                    U.S. Pat. No. 4,826,880                        [Patent Document 4]                    EP 0309187                        [Patent Document 5]                    EP 0844270 (Kokai)                        
However, even if the above prior art methods are used to carry out the agglomeration of the fine powders, there are problems that the operation of finely cutting the hydrous material of the water-absorbent resin or pulverizing it is difficult, and also there are problems that, in the case where the hydrous material of the water-absorbent resin in a high-water-content state is obtained, its drying takes high energy costs, and further there are problems of the thermal deterioration of the water-absorbent resin during the drying. In other words, in the case where the agglomeration strength of the fine powder is enhanced, water needs to be used in a large amount, so there are problems of the increase of costs and the thermal deterioration in addition to the workability deterioration. Hereinafter, further problems are described.
A water-absorbent resin has a property for surfaces of its particles to come to have the tackiness if it absorbs water. This tackiness depends on the water content of surfaces of the water-absorbent resin powder, and strong tackiness occurs in a certain water content range. The water content (ratio of water to the total weight of the water-absorbent resin and water) in this range where the tackiness is strong is in the range of 20 to 50 weight %. Above all, near 30 weight %, very strong tackiness occurs. This tackiness enables the agglomeration of particles of the fine powder.
As to the prior art methods, in nearly all cases, in order to make the water content of the hydrous material of the fine powder of the water-absorbent resin uniform, time is spent in mixing the fine powder of the water-absorbent resin with water, or the resultant hydrous material of the water-absorbent resin is left static for a definite time, thereby making the water content uniform. Examples of the shape of the above-shown hydrous material of the water-absorbent resin include a sheet shape and a particulate shape.
However, in the case where the surfaces of the hydrous material of the water-absorbent resin, which has been formed as a result of the agglomeration and integration of the fine powder and of which the water content has become uniform, have the water content in the range where the tackiness is strong, it follows that these surfaces exhibit the strong tackiness. Therefore, in the process in which a particulate agglomerate water-absorbing material is produced by subjecting the hydrous material of the water-absorbent resin to the steps such as of conveyance, fine cutting, and pulverization, a part of the hydrous material of the water-absorbent resin adheres to such as inner surfaces of treatment apparatus and thus can cause the poor operation of the treatment apparatus (e.g. increase of the load in operation of the apparatus, stop of the apparatus due to the overload). In particular, as the hydrous material of the water-absorbent resin adheres increasingly to such as inner surfaces and stirring-blades of mixers, cutting blades of fine cutters, and extrusion perforations of extruders to thus lead to the increasing growth (accumulation) of its adhered material, the efficiency of the fine cutting decreases, and the resistance during the extrusion increases, so that the treatment efficiency unfavorably decreases much.
On the other hand, in the case where the water content of the hydrous material of the water-absorbent resin is beforehand increased to not lower than 50 weight %, favorably not lower than 60 weight %, in order to enhance the treatment efficiency, the surface tackiness of this hydrous material may decrease. However, the decrease of the surface adhesive strength means that the binding between fine powder particles becomes weak. In other words, the deterioration of the shape-retaining ability and of the strength as to the formed hydrous material of the water-absorbent resin is caused, so that the handling rather becomes difficult.
Furthermore, because the final water-absorbent resin product is often supplied in a dry state, the contained water must be removed. Therefore, the hydrous material of the water-absorbent resin having been obtained in a high-water-content state takes a great deal of labor and time to remove water from. In other words, because the energy costs needed for the heat-drying increases unfavorably, the above is a method difficult to consider from the establishment of an efficient production process as originally aimed at. In addition, there has also been a case where the long-time drying causes the thermal deterioration of the water-absorbent resin. Thus, a method has also been proposed in which: in order to simplify particularly the drying, the amount of water being used is reduced to thus agglomerate the fine powder under conditions where the water content is in the range of several weight % to ten and several weight %. However, in the case where the water content is low, this method has had problems that the agglomeration strength is very weak.
Furthermore, it is also possible to consider carrying out the operations such as of fine-cutting and pulverization after having sufficiently dried the resultant the hydrous material of the water-absorbent resin to thereby get rid of the surface tackiness. However, depending on the agglomeration strength of the fine powder (binding force between fine powder particles), if the above operations are carried out after the drying has been completely made, then a fine powder unfavorably tends to newly be generated when the fine-cutting or pulverization is carried out.